1. Field of the Invention
The present invention relates to a magnetic sheet with stripe-arranged magnetic grains in which magnetic grains are arranged in a predetermined stripe-shaped pattern such as a linear or lattice-shaped pattern. In addition, the present invention relates to an RFID magnetic sheet used in an RFID (Radio Frequency Identification) system and an electromagnetic shielding sheet used in display devices etc., which are application examples thereof.
2. Description of Related Art
Conventionally, there are known applications, such as an RFID magnetic sheet or an electromagnetic shielding sheet, for a thin magnetic sheet in which a magnetic material layer is provided on a flexible substrate.
First, the conventional RFID magnetic sheet will be described. In recent years, it is becoming more common to utilize an RFID system, which is an automatic recognition technology that uses an electromagnetic field of a specific frequency emitted from an external device as a signal carrier, and performs communication of ID (Identification) information and various data with an external device. IC telephone cards, electronic tickets, and electronic money cards are examples of non-contact type IC cards that use an RFID system, and recently this RFID is also being equipped and used in mobile telephones.
In the case of equipping and using an RFID system on a mobile communication apparatus such as a mobile telephone, it is necessary to secure a communication distance, and, because of this, the elimination of influence by magnetic path obstacles is sought. More specifically, there is the drawback that when metal is adjacent to an RFID antenna in the RFID system, communication becomes impossible. In particular, in the case of using an electromagnetic signal of a high frequency such as 13.56 MHz, problems in the RFID system caused by this drawback are significant. In order to solve this problem, it is common to provide the RFID antenna with a magnetic sheet including ferrite with high magnetic permeability. (See, for example, JP 2004-227046A)
FIGS. 1 through 3 will be referred to in describing the problem of not being able to communicate when metal is adjacent to the RFID antenna.
FIG. 1 is a cross-sectional view showing the operation of a typical RFID system 100. In the RFID system 100, an RFID antenna (tag antenna) 102 is attached to an IC tag 101, which is one example of a non-contact type IC card, and to a reader/writer 103 is attached a reader/writer antenna 104. When communicating, the IC tag 101 is positioned close to the reader/writer 103. From the reader/writer antenna 104 of the reader/writer 103 a magnetic flux loop 105 is generated. RFID wireless communication between the IC tag 101 and the reader/writer 103 is made possible by a magnetic flux loop 105a that passes through both antennas (the tag antenna 102 and the reader/writer antenna 104). FIG. 1 shows a schematic view of the communication state in the case where there are no metal products in the vicinity of the IC tag 101.
FIG. 2 schematically shows the communication state in a RFID system 110 like in FIG. 1, in the case where there is a metal product 106 near the IC tag 101. In this case, an eddy current is generated in the metal product 106 positioned in the vicinity of the IC tag 101 by a magnetic field from the reader/writer 103. Further, a magnetic field (demagnetizing field) 107 generated by this eddy current cancels a magnetic flux loop 105b needed for communication. Therefore, communication becomes difficult.
Consequently, when the metal product 106 is adjacent to the IC tag 101, a measure for arranging a magnetic sheet 108 between the IC tag 101 and the metal product 106 is implemented, as shown in FIG. 3. The magnetic sheet 108 includes ferrite with high magnetic permeability, so a magnetic flux loop 105c can be aggregated on the magnetic sheet 108. As a result, it is possible to inhibit the generation of an eddy current in the metal product 106 and improve the communication distance.
The effect of the magnetic sheet 108 will be described in further detail with reference to FIGS. 4A and 4B. First, in the case where there is no magnetic sheet as shown in FIG. 4A, a magnetic flux loop 105d is lost under the influence of a magnetic field (demagnetizing field) that is generated by an eddy current in the metal product 106 that is in the vicinity of the IC tag (IC tag antenna 102) by the magnetic field from the reader/writer. As a result, a magnetic field needed for communication is canceled.
On the other hand, when the magnetic sheet 108 is used as shown in FIG. 4B, a magnetic flux loop 105e is aggregated on the magnetic sheet 108 because the value μ′ (real part) of the magnetic permeability of the magnetic sheet 108 is high. Furthermore, because the μ″ (imaginary part) of the magnetic permeability of the magnetic sheet 108 is low, the magnetic flux loop 105e flows without magnetic loss. As a result, the communication distance can be improved.
However, the magnetic field 108 including ferrite has the following problems. First, in the case of a magnetic sheet in which ferrite is dispersed in a resin, due to the magnetic sheet having a configuration in which magnetic material (ferrite) is dispersed in the resin, there is an upper limit to the effective magnetic permeability μ. In the case of a configuration using a sintered body of ferrite (ceramic) instead of dispersing ferrite in the resin, the effective magnetic permeability μ increases, but that magnetic sheet becomes brittle because it is a sintered body. Moreover, the magnetic sheet of that sintered body is also limited in how thin or flexible it can be made.
In order to balance flexibility with a high magnetic permeability μ, the present inventor considered using a metal magnetic material as the magnetic sheet. This kind of magnetic sheet has a higher μ than when ferrite is used, but generates an eddy current because it is electrically conductive. Thus, obstruction of communication results. Therefore, it is realistically difficult to use metal magnetic material as the magnetic sheet.
Next, the conventional electromagnetic shielding sheet will be described. Together with the recent increased use of various electrical equipment and electronic applied equipment, electromagnetic noise interference also is increasing. Noises are classified into conduction noise and radiation noise, and there is a method of using a noise filter as a measure against conduction noise. On the other hand, as a measure against radiation noise, since it is necessary to insulate a space electromagnetically, methods that have been adopted include making the casing of a metal body or a highly conductive body, inserting a metal panel between two circuit boards, or wrapping the cable with metal foil.
These methods can be expected to have an electromagnetic shielding effect in a circuit or power block, but, on the other hand, could not be applied to shielding of electromagnetic waves generated from the front surface of displays such as CRT, PDP, liquid crystal, and EL, because of being opaque.
An EMI shield function of 30 dB or greater at 1 GHz is desired for shielding of electromagnetic waves generated from a display surface, and particularly stringent standards are required to be met for PDP displays which are aimed at family TVs. In addition, good visible light transmittance is desired in the electromagnetic shielding sheet for display. There have been proposed several methods for balancing electromagnetic wave shielding and transparency.
For example, JP H10-41682A discloses an electromagnetic shielding sheet in which a geometrical configuration formed with conductive material is provided on the surface of the transparent plastic base material. JP 2000-323891A discloses an electromagnetic shielding sheet in which an adhesive layer is laminated on transparent base material, and a conductive layer of a geometrical configuration is buried in this adhesive layer. JP 2000-124662A discloses an electromagnetic shielding sheet including a conductive layer formed by applying, on a transparent conductive coating, a coating composition in which chain aggregates of metal microparticles have been dispersed. Typically, types such as a highly transparent film provided with a sputtered metal or a highly transparent film provided with a metal mesh are commonly used.
However, the above-described conventional electromagnetic shielding sheet cannot be said to have sufficient electromagnetic wave shielding properties. In order to solve this problem, the present inventor considered manufacturing an electromagnetic shielding sheet having high magnetic permeability and good electromagnetic wave shielding properties, by arranging a needle-shaped crystal indicating the characteristics of anisotropic soft magnetic material. However, the effect of the anisotropic needle-shaped crystal is reduced remarkably when its arrangement is made random, so there is the need to control the arrangement of the needle-shaped crystal.